Protocol assisted switched diversity of antennas

ABSTRACT

Fading is countered in a WLAN or similar system to maintain QoS by a fusion of switched diversity and protocol based redundancy using time spaced transmission bursts to improve the performance of radio receivers. This redundancy may be enhanced by an overlay of space-time coding of the transmission bursts.

FIELD OF THE INVENTION

[0001] This invention relates to improving the reception of digitalradio signals with reduced-complexity radio receivers, such as thosewhich may be used with handheld multimedia communication devices inwireless LAN applications. It particularly relates to radio receiverswith multiple antennas. In particular, the present invention uses aprotocol assisted switched diversity antenna system.

BACKGROUND OF THE INVENTION

[0002] Wireless LANs (Local Area networks) differ from wired LANs inthat various radio transmission channels may interfere with one another.Indeed signal propagation is further subject to many variations insignal strength due to such factors as multipathing a result of thevarious propagation paths a wireless signal may experience and otherfactors that interfere with a clean signal. Diversity is one means ofdealing with these various effects.

[0003] Antenna spatial diversity is one of the most powerful techniquesfor improvement of radio channel quality. The technique assumes thatradio signals arrive at antennas displaced in space via substantiallydifferent propagation paths when a multipath (Rayleigh) propagationenvironment is present. Thus, the signals arriving at each antenna aresubstantially decorrelated (provided large enough antenna spacing isused), and impairments which may affect each of them due to multipathfading are mostly non-overlapping in time and frequency. Using the twosignals in combination, with an appropriate combining technique based ona quality metric (e.g. received signal strength), can allow bettercommunication quality to be sustained.

[0004] In order to capitalize on the full value of antenna diversity, itis conventional to implement duplicate receivers for each antenna path.Although this “combinational” diversity approach is very effective, itcan be costly and difficult to implement in a low power environment. Asimpler approach, using a smaller number of receivers than the number ofantennas is called “switched diversity”. In these implementations, thereceiver uses one of the antennas to recover the desired signal whilemonitoring the quality metric. If the quality metric falls below anacceptable level, an RF switch is actuated to connect the receiverinstantaneously to a different antenna.

[0005] Usually switched diversity usage is relegated to analogtransmission systems (e.g. analog cellular) or digital systems, whichcan accommodate retransmission of unacknowledged or negativelyacknowledged messages. For systems in which the radio channel remainsstationary for an acceptable interval, switched diversity may be appliedusing a large number of antennas. Conventional switched diversitysystems, however, do not cooperate with MAC protocols since theswitching of the antenna is autonomous at the receiver.

[0006] Wireless LANs are now being contemplated for delivery oftime-bound multimedia communications in addition to their current usefor non-time-bound data. Protocols have been developed for providingscheduled, non-conflicting time intervals for transmission of multimediapackets whose latency requirements cannot accommodate conventionalretransmissions for error correction. Because delays caused byack/nak-directed retransmission cannot be tolerated, one must seek othermeans to reduce error rate. Forward error correction coding is usuallyused for such purposes, but its use may incur large coding overheads inthe case of multipath propagation environments where a significantnumber of symbols may be eliminated by a fade at a single antenna.

[0007] Although combinational diversity is an attractive means ofreducing error rate in multipath environments, wireless LAN clientsfrequently require low dissipation and small (PCMCIA) form factors, andare less able to support the complexity and cost of multiple receivers.Eventually, VLSI techniques will succeed in meeting the size, cost, anddissipation requirements for these clients. However, it would beadvantageous to have a means by which the switched-diversityarchitectures in use for today's wireless LAN receivers could beutilized to provide the necessary improved BER performance inassociation with software-based protocol and coding techniques.

SUMMARY OF THE INVENTION

[0008] An exemplary diversity system involves use of a novel fusion ofswitched diversity with protocol-based transmission redundancy anderror-correction coding to improve to improve the performance of radioreceivers. A specific embodiment disclosed herein uses a single receiverwhich may be connected to more than one antenna via an RF switch. Incontrast to conventional switched-diversity operation, the switch iscontrolled, not by signal strength or other metric, but ratherincremented by the sequence number of a series of scheduled packetbursts which are prescribed by a QoS protocol. The message itself isrecovered from the series of packet transmissions, each displaced intime. Each transmission may be coded in such a way as to provide acombination of error-correction coding and user data, providing atrade-off opportunity between radio resource use and error rate. Anexample of a coding scheme which may be utilized is space-time codes.

[0009] In one particular embodiment a receiver of a base station havingtwo decorrelated antennas is enabled to operate with a fusion ofswitched diversity reception and protocol based redundancy using timespaced transmission bursts, each containing the same message, to improvethe performance of radio receivers (i.e., including mobile receivers) ina WLAN. The protocol works with the antenna switching process to providethe best signal reception.

[0010] In another embodiment space-time codes are used to spread themessage information over the two transmitted bursts separated in timewithin a PCF control frame such as defined in the WLAN 802.11 standard.This advantageously allows reduction of radio resource use as comparedwith the above redundant transmissions. The robustness of the operationis maintained.

[0011] These embodiments may include a capability of notifying atransmitting end from a receiving end of the transmission that a clientreceiver is capable of protocol assisted switched diversity operations,including the number of antennas and receivers available for reception.

[0012] In another aspect of the invention, where enough bursts have beenreceived successfully to reconstruct the transmitted message (via checksum or other error detection technique), an option is given a receivingend of the system to acknowledge correct receipt of the message. Atransmitting end to may use this acknowledgement to cause thetransmitting part of the system to cease sending diversity bursts. Thispermits a significant conservation of system radio resources.

DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a schematic of an exemplary receiver using two antennasto achieve spatial diversity; and

[0014]FIG. 2 is a graph of control protocol frames used in an exemplarytransmission media.

DETAILED DESCRIPTION

[0015] In an examplary two-antenna, one-receiver embodiment as shown inFIG. 1, receiver (100) is connected to antennas (101 and 102) by an RFswitch (103). The receiver consists of a conventional analog RFprocessing (104), signal strength measurement (105) and digitaldemodulation (106) facilities. The digital output of the receiver,consisting of message symbols and associated signal strength values, isconnected to a buffer memory arrays (107 and 108). Control of RF switch103 is accomplished with the aid of microcomputer (109) which executes astored program implementing the MAC protocol. Based on the state of theprotocol, the RF switch control will be operated in the conventionalmode (switching based on an RF signal strength quality metric asdescribed above), or in the multiple burst mode (where it is switched toensure that a particular burst is received by a particular antenna.

[0016] For the purposes of this example, the base station will beassumed to have at least combinational diversity reception, and may alsosupport transmit diversity. In operation, the method works as follows:Operating in the conventional switched-diversity mode, the clientreceiver has acquired the RF channel and has selected an antenna whichis delivering acceptable signal strength. By receiving the channel for aperiod of time, the receiver has allowed the MAC microprocessor tosynchronize with the base station transmissions and to cooperateaccording to protocol rules for channel access. The MAC communicates tothe base station that its hardware configuration supportsswitched-diversity QoS improvement as part of its session-accesspreamble, during which it requests a specific stream QoS type.Ordinarily, the switched-diversity QoS option would be engaged only forhigh-priority traffic in a priority-based scheduling system, or for thetraffic requiring the highest end-to-end QoS performance in aparameter-based system. The balance of the example will assume that the“parameterized-QoS” mode is used (see FIG. 2).

[0017] The QoS mode illustrated in FIG. 2 uses a protocol set of aproposed 802.11e standard. In this protocol set, a station (STA/client)communicating with an access port (AP) may only use the wirelesstransmission medium during specified periods of time. These specifiedperiods of time include Contention Free Periods (CFPs), ControlledContention intervals (CCI) and Resource Reservation (RR) frames andContention Control (CC) frames. A controller grants the transmissionmedium for use by RR frames by transmitting a CC frame. Only RR framesare transmitted during a time period specified by the CCI frame. The RRframes define the needed Bandwidth of the designated QoS. The CC framesdesignate parameter for the CCI. Further details of this procedure arecontained in the 802.11 proposal of the IEEE.

[0018] When the base station determines that downlink (to the client)traffic has arrived from the network, it schedules a series of bursttransmissions (in this example, two). The base station initiates messagetransmission by issuing a polling request, followed by the first packetburst. The first burst, containing the message, will be receivedexclusively on the antenna which has been in use (the RF switch remainsset for the duration of the burst). While the burst is being received,the receiver's output (soft symbols and signal strength values) isstored sequentially in buffer 107. At the conclusion of the basestation's transmission, the client transmits a polling response,followed by any uplink (to the base station) traffic it may have tosend. The microprocessor, which has been adhering the protocol,immediately causes the RF switch to connect the alternate antenna to thereceiver, in preparation for reception of the second burst, containingthe same message. At some later moment in the current superframe or asubsequent superframe, the base station transmits a second pollingrequest and the second packet burst. This burst is received exclusivelyusing the second antenna; the receiver output is similarly storedsequentially in buffer 108.

[0019] Following receipt of the second burst, the microcomputer extractsthe first symbol of the first burst from buffer 107 with itscorresponding signal strength value. Likewise, it extracts the firstsymbol of the second burst from buffer 108, with its correspondingsignal strength value. Using a combination of symbol decoding, errorcorrection decoding, and combining based on the signal strength metric,the desired message is extracted. Techniques such as Maximal RatioCombining (MRC), well known to those skilled in the art, could be usedfor this purpose.

[0020] Since the method uses implicit time-displaced redundancy incooperation with antenna diversity, the quality of the recovered messageis comparable to conventional combinational diversity if the channel isstationary during the interval which includes both bursts and the burstscontain exactly the same information. If the channel is not stationaryover this interval, the method produces a form of space-time spreading,which may provide improvement over combinational diversity methods.

[0021] Using identical (duplicated) transmission in the bursts usestwice the radio resource than would be required with a combinationaldiversity system. For time-bound (fixed duration) material which usesrelatively short packets (e.g. digital voice), this increase in resourceusage would undoubtedly be an acceptable trade-off for increased qualityand hardware simplicity. For high throughput (large packet) traffic,total redundancy of packets may constitute unacceptable overhead.Accordingly, by use of appropriate coding techniques (e.g. space-timecodes) it is possible to trade off quality improvement for radioresource usage by partially or completely spreading the message acrossthe bursts. Such a strategy allows flexibility between BER improvementand channel occupancy. Space-time codes may be used for improvingperformance in a wireless environment and when combined with protocolswitched diversity significantly improves transmission efficiency overthat of the protocol switched diversity alone.

[0022] Additions to the invention may include adding an ability of thesystem to communicate from a terminating end to a transmitter to specifythe receiver capability to perform protocol assisted diversity operationand include the number of antennas and radio receivers that theterminating end has.

[0023] In another arrangement, the receiving end may notify atransmitting end promptly if a message is successfully received,allowing reconstructing of the transmitted message, before a subsequentburst is received. This allows action to cease further transmission ofbursts related to this particular message thereby resulting in aconserving of radio resources in the system.

[0024] Those skilled in the art may devise many variations of theseschemes without departing from the spirit and scope of the invention. Ahybrid architecture consisting of switched-diversity and combinationaldiversity elements may be used (e.g. four antennas, a 4×4 switch matrix,and two receivers) to provide significantly improved performance overconventional two-branch combinational diversity.

What is claimed is:
 1. A radio receiver comprising first and second antennas connected to RF processing circuitry by an RF switch; an RF switch control switched incrementally in response to a sequence of scheduled packet bursts.
 2. The radio receiver of claim 1, wherein: the RF switch control schedules sequence bursts prescribed by a QoS defined by a MAC protocol.
 3. The radio receiver of claim 2, wherein: a MAC processor is synchronized with transmission of a base station.
 4. The radio receiver of claim 1, wherein: the antennas are switched so that each antenna receives a related packet burst.
 5. A method of maintaining a controlled QoS in a wireless communication system, comprising steps of: receiving communications from a transceiver at a transmission station by wireless transceivers at receiving stations having switched protocol diversity reception operational modes; communications being formatted as multiple packet bursts; enabling a first antenna to receive a first packet burst; enabling a second antenna to receive a second packet burst; recording the received bursts as soft information in a storage medium; combining the soft information from the first and second bursts into a single message.
 6. The method of claim 5 wherein: each packet burst contains a same complete message.
 7. The method of claim 5 wherein: each packet burst contains a portion of a space-time coded message spread across the first and second packet bursts.
 8. A method of achieving a QoS control in a wireless LAN communication system, comprising steps of: transmitting a message contained within a plurality of packet bursts occurring at spaced time intervals; receiving the packet burst individually at a plurality of antennas.
 9. The method of claim 8 wherein; each of the plurality of the antennas is connected to a radio receiver at separate times relative to other receiving antennas.
 10. The method of claim 8, wherein: including a complete message within each packet burst.
 11. The method of claim 8 wherein: a message is spread across the plurality of packet bursts by space-time coding.
 12. The method of claim 8 wherein: the process of signal transmitting combines a protocol with signal processing.
 13. A communication system for coupling a transmitter and a receiver adapted for receiving at least first and second signal bursts by first and second antennas respectively, and responding to the two signal bursts to communicate a single unified message at the receiver; whereby: the first and second signal bursts are sequentially separated in time; the first and second antennas are sequentially enabled to communicate to storage at the receiver; enabling a representation of the unified message by responding to the first and second signals.
 14. The communication system of claim 13, wherein: the first and second signal bursts are identical packets of a common message.
 15. The communication system of claim 13, wherein: the first and second signal bursts are each a part of a space-time coded message spread across two bursts; and a common message is derived from the sequential signal bursts received by the first and second antennas.
 16. The communication system of claim 13, wherein: enabling includes retaining the first and second signal bursts in a storage medium and processing to deliver the single unified message.
 17. The communication system of claim 13, wherein: deriving the common message includes selecting a message from one of the receiving antennas.
 18. The communication system of claim 13, wherein: deriving the common message includes decoding a space-time coded signal spread across and received by both the first and second antennas.
 19. The method of claim 8, including a further step of: notifying a transmitter at a transmitting end by a receiving end of the number of antennas and radio receivers at the receiving end.
 20. The method of claim 8, including a further step of: a receiver notifying a transmitter that it accepts and responds to protocol-assisted diversity operations.
 21. The method of claim 8, including a further step of: upon reconstruction of a received message sending a message to the transmitting end to cease further message bursts. 